Alphanumeic call signs are already used around the world [0], it's just that the US (like many other aviation fields) is behind the times.
reply[0] https://www.flightradar24.com/blog/aviation-explainer-series...
The format is SSIM defined by IATA - the international association of transport airlines. It specifies two letter airline code followed by four digit flight.
replyI'm sure various ATC systems also assume this.
Changing it would be an industry-wide effort.
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